The present invention relates to a method of logging a rod-pumped well; more particularly, to a method of logging a rod-pumped well with a resilient-coated cable which provides protection to the interior production tubing to minimize scarring and damage.
Historically, wells completed and requiring artificial lift of the oil from the production zone were rarely logged because of the high cost of removing the rod pump completion, including the production tubing, and replacing both again after logging. New measurement equipment in logging services has become available which makes logging existing wells under rod-pump artificial lift systems more desirable and more easily accomplished. The time required to move a work-over rig on a well site and pull the entire production string before commencing logging has generally made such efforts uneconomic.
Logging of producing wells permits adjustment of production rates, reservoir studies, and other useful information to be gathered to maximize recovery from the well and the surrounding reservoir.
In oil wells requiring artificial lift means, one of the primary means is the rod pump where a subsurface pump and the production tubing work together to lift the oil from the well bore. One configuration is the stationary barrel pump in which the barrel remains fixed while a plunger moves inside it. As the sucker rods pull the plunger up, the hydrostatic head of the tubing fluid pushes a traveling valve (a ball and seat valve) closed and opens a standing valve. As the plunger continues upward, the pressure between the valves is low, and bottom hole pressure opens the standing valve and pushes liquid into the barrel. As the rods begin to move downward, the standing valve closes immediately. With continued downward movement, the pressure between the valves increases until this pressure exceeds the tubing hydrostatic head and opens the traveling valve, allowing liquid between the valves to move above the traveling valve. This portion of liquid is lifted as the next upstroke begins.
Another subsurface pump is the traveling barrel pump. The operation of this pump is similar to the operation of the stationary barrel pump except that the traveling valve is attached to the barrel. The pump may be a complete unit which is attached to the rod string. The pump is lowered into the tubing and attached to a seating nipple by plastic, fiber, or mechanical cups called hold-downs. In some types of rod-pumping systems, one of the bottom tubing joints is the pump barrel. The standing valve, attached to the traveling valve for installation and detached for operation, is lowered and mounted in the seating nipple. Such pump is called a tubing pump.
Irrespective of the type of pump deployed, the hold-down may be located and locked into a seating nipple on the production tubing either above the pump or below the pump. In deep wells, for example, a hold-down at the bottom of the insert pump experiences the hydrostatic head of tubing liquid on both the outside and inside of the barrel and a thin-walled barrel is acceptable. However, solids pumped with the liquids will settle around the pump and possibly wedge it in the tubing. On the other hand, a top-down pump can be used to wash away solids continuously, but a much heavier barrel is required to contain the great difference in pressure between inside and outside without bursting. A compromise is the use of both top and bottom hold-downs, which combine the advantage of both but these require special seating-nipple arrangements.
The selection of tubing is critical for an operator of such wells since during operation the weight of the tubing liquid is transferred from the rod string to the tubing and back. The weight of the tubing liquid is several thousand pounds and the transfer of this much liquid can make the tubing string stretch several feet and rebound on each stroke. This stretching action stresses the tubing and can cause tubing failure. Operators are therefore very wary of allowing anything to be used on the interior surface of the tubing string that may accelerate the fatigue/failure cycle. Further, the integrity of the pump body and the seats requires any service work performed avoid damage to the interior surface of the pump body.
A through-tubing logging method has long been needed for a rod-pumped well system. The present invention permits well logging in a fraction of the time necessary using prior methods thereby saving substantial expense for rig and crew time.